This invention relates to a crankshaft for a multi-cylinder engine and more particularly to a crankshaft construction having high strength and light weight.
It is well known that a crankshaft has a certain resonant frequency and it is desirable to design a crankshaft so that this resonant frequency does not fall within a range where it will be excited during the normal engine running speed. The resonant frequency of a crankshaft may be expressed by the following equation: ##EQU1## wherein: F=the resonant frequency,
K=the spring constant of the crankshaft, and PA1 I=the inertial mass of the crankshaft.
The resonant frequency is related to the resonant crankshaft angular velocity by the following formula: EQU .function.=1/2.pi..omega.
wherein .omega.=the resonant crankshaft angular speed.
Therefore, the resonant crankshaft angular speed may be expressed by the following equation: ##EQU2##
In order to make the crankshaft such that the resonant crankshaft angular velocity is above the normal speeds encountered by the associated engine, it is necessary to either make the spring constant very large or the inertial mass very small. Making a large spring constant requires the use of a very massive crankshaft. Such massive crankshafts are, however, undesirable because they take up large space and can cause the overall engine to become unduly large. On the other hand, a substantial reduction in the inertial mass means that either the crankshaft and/or its associated flywheel become unduly light. If the flywheel is too light, then it will not serve its purpose in providing smooth running and damping of the irregular firing impulses. On the other hand, if an attempt is made to reduce the resonant crankshaft angular velocity to a speed below the normal engine speeds, either the spring constant K must be substantially reduced or the inertial mass I must be made very large. If the spring constant is made very small, then the crankshaft tends to become too weak. On the other hand, if the inertial mass I is increased substantially, the mass of the flywheel adversely affects engine performance.
It is, therefore, a principal object of this invention to provide an improved crankshaft construction for a multiple cylinder internal combustion engine.
It is a further object of this invention to provide a crankshaft construction for a multiple cylinder internal combustion engine wherein the resonant frequency of the crankshaft lies outside of the normal engine running speeds and wherein the mass of the flywheel is kept at reasonable limits and the crankshaft itself may have the appropriate strength but not be unduly heavy.